NMR magnet device for solution analysis and NMR apparatus

ABSTRACT

The split type magnet device for high-sensitivity NMR apparatus to be used for solution analysis of the present invention provides a direction which generates a remarkably uniform magnetic field at the center portion of the magnet device used for determining the sample. The magnet device for NMR apparatus comprises first multilayer coils and second multilayer coils which face to each other with a predetermined distance being provided therebetween, each of pairs of the first and the second coils being substantially coaxial with respect to a central axis, each of layers of each of the first and the second multilayer coils having at least one coil, wherein an energizing current of at least one of the coils constituting an innermost layer of each of the first and the second multilayer coils is in a minus direction if an energizing current of the coil used for generating a main magnetic field for NMR detection in the vicinity of a center portion of the apparatus is in a plus direction.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a nuclear magnetic resonance analyzing apparatus and, particularly, to a magnet device for generating a uniform magnetic field.

[0002] Rapid progress has recently been made in the field of organic analysis using the nuclear magnetic resonance. In particular, atomic structures of organic compounds such as proteins can now be efficiently analyzed by the combined use of the nuclear magnetic resonance and the superconducting ferromagnetic technology. An object of the present invention is to provide an NMR spectrometer which is necessary for analyzing atomic structures and interactions of protein molecules in a solution (the solution is prepared by dissolving minute amounts of proteins into a liquid). The NMR spectrometer of the present invention is a special energy spectrometer which is different from a medical MRI diagnosing apparatus having an image resolution of millimeter order to be used for human tomography. Such special energy spectrometer must have magnetic field intensity which is higher than that of the medical MRI diagnosing apparatus by one digit, a magnetic field uniformity which is higher than that of the medical MRI diagnosing apparatus by four digits, and a stability which is higher than that of the medical MRI diagnosing apparatus by three digits, and design engineering and manufacturing technology thereof are totally different from those of the medical MRI diagnosing apparatus. Details of the conventional high-resolution nuclear magnetic resonance analyzing apparatuses can be found in “NMR of Proteins” which was written by Yohji Arata and published by Kyoritsu Shuppan in 1996. Recent inventions relating to typical equipment component to be utilized for applying NMR to the protein analysis include: Japanese Patent Laid-open No. 2000-147082 which discloses an invention relating to the superconducting magnet, i.e., multilayer air-core solenoid coils; U.S. Pat. No. 6,121,776 which discloses a birdcage superconducting detection coil; Japanese Patent laid-opens Nos. 2000-266830 and 6-237912 each of which discloses a signal detection technique using a conventional saddle-like coil or a birdcage coil; and so forth. According to the above literatures, all of the conventional high sensitivity nuclear magnetic resonance analyzing apparatuses for protein analysis include a superconducting magnet device which uses solenoid coils combined which generate a magnetic field in a vertical direction, thereby irradiating a sample with an electromagnetic wave of 400 to 900 MHz to detect a resonance waveform produced from the sample by the use of the saddle-like or the birdcage detection coil. In some cases, the S/N parameter is improved by the use of a detector which is cooled to a low temperature in order to reduce the thermal noise in the detection of signals as disclosed in the above-mentioned United States Patent.

[0003] In the past, the sensitivity of the high sensitivity nuclear magnetic resonance apparatuses has been improved through the use of methods of improving intensity at the center portion of the magnetic field of the superconducting magnet without changing a basic system structure of the apparatus including an antenna, a magnet, and so forth. Therefore, although the maximum NMR detection sensitivity which has heretofore been reported was achieved by an NMR apparatus of 900 MHz which uses a large superconducting magnet of FIG. 8 having a center magnetic field intensity of 21.1 Tesla, the basic structure of the NMR apparatus is not any different from that of the prior art disclosed in Japanese Patent Laid-open No. 2000-147082. For better understanding of the basic structure, a sectional perspective view of the superconducting coils of the NMR apparatus is shown in FIG. 9 wherein a virtual central axis is indicated. In the protein analysis using solution, the increase in the intensity at the magnetic field center has effects of improving the sensitivity and clarifying separation of a chemical shift.

[0004] The sensitivity improving effects depending on the shapes of the detection coils are mentioned in page 326 of “Book of NMR”, which was written by Yohji Arata and published by Maruzen in 2000, where the solenoid coil is advantageous as compared with the saddle-like coil or the birdcage coil in various aspects as described below.

[0005] For example, the solenoid coil is superior in controllability of impedance, filing factor, and RF magnetic field efficiency. According to the “Book of NMR”, however, it is actually impossible to wind solenoid coils around a sample tube which is placed perpendicularly with respect to the magnetic field in the conventional superconducting magnet structure and, in general, the solenoid coil has not been used in the cases where the sensitivity is crucial such as a case of determining a minute amount of protein dissolved in a solution. For a particularly exceptional case wherein the solenoid coil is used for determining a minute amount of sample solution with high sensitivity, a method which employs specially designed micro sample tube and probe combined with the solenoid coil is known.

[0006] As a particular example, a method wherein a high-temperature superconducting bulk magnet is magnetized in the horizontal direction to detect NMR signals using the solenoid coil is disclosed in Japanese Patent Laid-open No. 11-248810. Japanese Patent Laid-open No. 7-240310 discloses structures of a superconducting magnet and a cooling container suitably used for a typical NMR for eliminating restriction in the height of the apparatus. However, the method of achieving the detection sensitivity, which is required for analyzing proteins, and techniques of controlling the uniformity and the time stability of magnetic field have not yet been proposed.

SUMMARY OF THE INVENTION

[0007] Recently, the need for analysis of samples which contains proteins each having a low solubility in liquids has been grown with the increase in the need for investigations of proteins, and such growing need has brought about the necessity of improving sensitivity for detecting NMR. In order to adapt a nuclear magnetic resonance analyzing apparatus for the needs described above, it is necessary to improve the detection sensitivity without increasing the size of a sample space as compared with conventional apparatuses and, further, it is mandatory to provide a stability of a superconducting magnetic field for a long period of data integration time. The improved sensitivity contributes not only to the reduction in detection time but also to the reduction in sample amount, thereby achieving the effect of enabling the analysis of proteins having a low solubility to liquids. Thus, it is required that the NMR analyzing apparatus to be used for the protein analysis have particularly superior detection sensitivity and stability as compared with the conventional ones, and accurate and stable NMR signal detection over a long period of time such as a week or more must be assured. This is because a fluctuation in the magnetic field during the detection causes a peak of NMR signals to move, and such movement of peak in a detection of interaction can be confused with a movement of peak caused by the interaction or the unstable magnetic field. Further, if the magnetic field is non-uniform, desired peaks are overlapped to cause problems such as difficulty in distinguishing the interaction. Thus, the future NMR technology used for the purpose of analyzing proteins must be a novel technology which is not a simple modification of the conventional NMR apparatuses.

[0008] For example, specifications for a uniformity of a magnetic field provided in a typical NMR apparatus are 0.01 ppm in a sample space and 0.01 ppm/h by time stability. If this magnetic field is used in a typical proton NMR of 600 MHz, an allowable error is 6 Hz. However, for the analysis of interactions of proteins, the required space and time resolutions are not more than 1.0 Hz, preferably not more than 0.5 Hz.

[0009] The superconducting magnet and the detection coil must be optimized so as to achieve the above-identified uniformity and time stability of magnetic field. Therefore, the performances of the conventional NMR apparatuses are insufficient, and the required stability and uniformity are greater than those achieved by the conventional NMR apparatuses by one digit or more.

[0010] Since the improvements in sensitivity of the conventional NMR apparatuses have been achieved by intensifying the magnetic field, the apparatuses have become too large. Since a dedicated room was required for the apparatuses in view of the leakage magnetic field and the floor strength, problems such as poor installability have arisen. Also, increase in the cost of the superconducting magnet has been problematic. Further, in the improvement in sensitivity by the conventional technique, the intensity has reached to its upper limit of 21 T due to the restriction caused by the critical magnetic field of superconducting material. Therefore, in order to achieve a further improvement in sensitivity, there has been a demand for a new technique of improving detection sensitivity which does not rely on the magnetic field intensity.

[0011] High-sensitivity detection using the solenoid coil can be realized by the use of the special sample tube which contains a minute amount of a sample and the special detection probe as mentioned above; however, the high-sensitivity detection method is not suitable for analyzing a protein solution as little as about 10 cc. Further, according to the invention wherein a magnetic field is generated in the horizontal direction by the use of a ferromagnetic member and NMR signals are detected by the use of the solenoid coil, which is disclosed in Japanese Patent Laid-open No. 11-248810, the magnetic field less than 10 T is generated on the surface of the high temperature superconductor, while the magnetic field near the sample has an intensity of a several teslas at most. Therefore, it is impossible to achieve the magnetic field of 11 T or more, preferably 14.1 T or more, which is required for the protein analysis in a desired sample space, through the use of the invention disclosed in Japanese Patent Laid-open No. 11-248810.

[0012] Further, the invention disclosed in Japanese Patent Laid-open No. 11-248810 involves a difficulty in achieving a time stability of 1.0 Hz/time or less, which is required for the protein analysis, due to the magnetic flux creep of the superconductors. Further, the invention also involves a difficulty in achieving a magnetic field uniformity of 1.0 Hz or less of proton nuclear magnetic resonance frequency, which is required for the protein analysis, in a space having a diameter of 10 mm and a length of 20 mm, since there is the non-uniformity derived from a manufacturing process of the material of the superconducting bulk magnet.

[0013] Thus, developments in break-through technology to fit the needs for the protein analysis are in demand, while the improvement in sensitivity achieved by increasing the magnetic field intensity has reached the ceiling. Therefore, a novel technique for the improvement in sensitivity has been in demand.

[0014] To provide efficient, accurate analysis of interactions of proteins with low molecular weight molecules such as a substrate and ligand in a solution, for which the need will hereafter be grown, it is desirable that the analysis be conducted by using an appropriate amount of sample with a frequency of 600 to 900 MHz and an intensity at the magnetic field center of about 14 to 21 T, and that detection sensitivity and throughput be increased.

[0015] In general, in the apparatus of 800 MHz or more, the liquid helium of 4.2 K is depressurized and supercooled to 1.8 K in order to exploit the superconductivity to the utmost. Therefore, operation of the apparatus is complicated and very careful maintenance therefor is necessary.

[0016] Further, since the leakage magnetic field is increased with the increase in the size of the magnetic device, a dedicated room is usually required for the conventional NMR apparatus. In particular, from the standpoint of the installability of the apparatus, the conventional magnetic device has the problem of the leakage magnetic field which is increased in the vertical direction with the increase in the intensity at the magnetic field center. For example, since the conventional class 900 MHz magnet device generates a leakage magnetic field as long as 5 m in height, a room having a considerable ceiling height is required.

[0017] Thus, the conventional magnet device has the problem of increased cost for installation. In addition, a magnet portion of the conventional 900 MHz superconducting magnet has a diameter of 1.86 m and a height of a several meters as is disclosed in “IEEE. Transactions on Applied Superconductivity, Vol. 11, No. 1, p. 2438”.

[0018] An object of the present invention is to provide a novel nuclear magnetic resonance analyzing apparatus in which the detection sensitivity of NMR signals is improved by 40% or more at the frequency of 600 MHz (14.1 T) with respect to the conventional apparatuses in a state where an ordinary sample tube having a diameter of 5 to 10 mm and containing a sample solution of 30 mm is placed therein and in which a time stability and a space uniformity of the superconducting magnet required for the protein analysis are provided. The present invention shall not be intended to limit the operation temperature to 4.2 K in structure. It is possible to realize the ultimate performance through the application of the present invention and, depending on the mode of the application, the apparatus of the present invention may be operated with the conventional upper limit magnetic field intensity of 21.1 T, i.e., 900 MHz and 1.8 K. In this case, the apparatus of the present invention is allowed to improve the sensitivity by 40% as compared with the conventional apparatuses, thereby advancing the upper limit of the detection sensitivity for the first time in the art.

[0019] The inventors of the present invention have conducted intensive studies to find out problems of the conventional nuclear magnetic resonance apparatuses and solutions thereto. Since the existing nuclear magnetic resonance apparatuses have been developed from the standpoint of both cost and installability, NMR signals produced from a solution sample placed at the center portion of multilayer air-core solenoid coils having an excellent magnetic field uniformity have been detected by the use of a saddle-like or birdcage antenna. In the past, with the development in the NMR technology attained by the progresses in detection technique and analysis method, the detection sensitivity has been improved through the enhancement of intensity at the magnetic field center from a magnetic field intensity of less than 400 MHz without changing the basic structure. Further, use of the superconducting birdcage antenna for the reduction of thermal noise has recently been proposed. The inventors have conducted intensive studies on a technique for remarkably enhancing the signal intensity without changing the magnetic field intensity. As a result, we have found that the novel structure described below can solve the problems described above.

[0020] The invention enables the improvement in sensitivity by using a solenoid detection coil having a diameter of about 5 to 10 mm and a height of about 20 mm, with which an ordinary sample tube for NMR investigations can be used, in a sample space having a diameter of 5 to 10 mm and in a magnetic field of not less than 400 MHz, preferably about 600 to 900 MHz, which is suitable for solution NMR of a height of 20 mm.

[0021] In theory, it is expected that the sensitively of 1.4 ({square root}2) times that achieved by the conventional devices is achieved by the shape of the coil, and further improvement is expected by other factors. Also, the data integration time can be shortened to not more than half that of the conventional devices. The sample tube having the diameter of 5 to 10 mm is charged with a liquid sample until it reaches the height of 20 to 30 mm and inserted into the apparatus from above along the vertical direction. In order to detect the NMR signals with high sensitivity by the solenoid coil which is wound about a vertical axis, it is necessary to magnetize a magnetic field to be generated by the superconducting magnet in the horizontal direction and to make it possible to place the solution sample, which can readily be attached and removed, at the center portion of the magnetic field. Therefore, the structure of the superconducting magnet device of the present invention is different from that of the conventional devices which use the simple solenoid magnet, and thereby it is necessary that the superconducting magnet be a split type magnet having a pair of left and right magnets. It is necessary that the magnet to be used for the NMR apparatus generate a magnetic field of ppb-order uniformity in the sample space as mentioned above. A technique for achieving the uniformity of the magnetic field using the high magnetic field split type magnet of 14.1 T has not been proposed in the art. As shown in FIG. 10, the present invention is characterized in that a current conveyed by coils constituting the innermost layer among two sets of multilayer coils facing to each other with a predetermined distance flows in a direction reverse from that of a main magnetic field.

[0022] Referring to FIG. 10, the arrow indicated in FIG. 10 is a virtual central axis of coils 1 to 5 and 1′ to 5′ constituting the magnet. Since each of the coils is wound around a spool, which is not shown in FIG. 10, a section thereof typically has a substantially rectangular shape as shown in FIG. 10. Also, in order to facilitate fabrication of the magnet, it is desirable to reduce the number of spools. Therefore, as the coils 4 and 5 of FIG. 10, the coils having a substantially identical inner diameter are usually wound around an identical spool. In the present invention, the coils such as the coils 4 and 5 which have the substantially identical inner diameter, a substantially identical central radius or a substantially identical outer diameter as seen in their sectional views are referred to as coils forming an identical layer.

[0023] In order to generate a uniform magnetic field at a central part of the magnet where a sample is inserted, it is necessary to use the coils which generate a magnetic field in a direction reverse from that of a main magnetic field as the coils for constituting the innermost layer in the split type magnet of the present invention. Reasons for such necessity will hereinafter be explained.

[0024]FIG. 12 is a sectional view showing a two-dimensional axisymmetrical distribution of currents. As is well-known, the uniform magnetic field is generated in an inner space of the current distribution by setting a current I(θ) of an angle θ when the center axis is z axis to be a so-called cosine θ distribution as represented by the following expression.

I(θ)=A cos(θ)

[0025] In the above expression, A is a proportional constant. Generation of the uniform magnetic field by the use of a limited number of coils means discretization of the current distribution shown in FIG. 12. However, as it can be understood qualitatively, it is difficult to discretize the currents in a region between two dotted lines of FIG. 12 outside the region through the discretization of the unidirectional current distribution of FIG. 12 using the split type coils. It is possible to overcome the difficulty by using expressions for the magnetic field generated by an annulus current as follows. It is possible to represent z component of a magnetic field which is generated at the point P (r, θ) in an inner region of a sphere formed by the inscribing of the annulus current I to the annulus ring as shown in FIG. 13 by the following expressions. $\begin{matrix} {B_{z} = {\sum\limits_{n = 0}{A_{n}r^{n}{P_{n}\left( {\cos \quad \theta} \right)}}}} \\ {A_{n} = {\frac{\mu_{0}I}{2f^{n + 1}}\sin \quad \alpha \quad {P_{n + 1}^{1}\left( {\cos \quad \alpha} \right)}}} \end{matrix}$

[0026] In the above expressions, A_(n) represents the intensity of n-th order magnetic field. Since the magnet of the present invention is mirror-symmetrical with respect to the center plane, only the even-order magnetic fields should be considered due to the symmetry. It is possible to generate the uniform magnetic field by setting a sum of the second-order irregular magnetic fields to zero. Shown in FIGS. 14 to 16 are graphs in each of which the magnetic field intensity (A₂, A₄, and A₆, respectively) is plotted with respect to α. In the graphs, it is defined that μ₀I/2/f^(n+1)=1. As shown in FIGS. 15 and 16, since each of the higher-order magnetic fields takes positive and negative values with respect to α, the sum of the magnetic fields of all the coils can readily be set to zero if an arrangement and magnetomotive forces of the discrete coils are properly set. In turn, the second-order magnetic field shown in FIG. 14 becomes zero when tan α=2. This position is well-known as a Helmholtz coil. As is apparent from FIG. 14, when α is smaller than the α of tan α=2 (about 63.4 degrees), the second magnetic field takes only the positive value. As is apparent from FIGS. 1, 2, and 10, since almost all of the coils are placed at positions where α is smaller than the above-described α (about 63.4 degrees) in the split type coil arrangement, it is impossible to set the total of second-order irregular magnetic fields generated by the coils having a current in the direction (positive direction) identical with that generating the main magnetic field to zero. Therefore, it is necessary to use coils having a current in the direction reverse (negative direction) to that generating the main magnetic field, and it is desirable that a portion at which the coils are to be arranged be such that the radius of each of the coils is smaller than those of other coils. Reasons for the desired arrangement will hereinafter be explained. The intensity of an irregular magnetic field generated by the annuls current I shown in FIG. 13 is inversely proportional to f^(n+1) according to the above-identified expressions. Therefore, a smaller radius of the annulus current is more advantageous for generating A₀, i.e., a negative second-order irregular magnetic field, without impairing the intensity of the main magnetic field. In principle, it is possible to set the second-order irregular magnetic field to zero by the use of negative current coils having a large radius; however, in view of the above reasons, such negative current coils are unrealistic as to be used for making up the high magnetic field generating magnet, which is the subject of the present invention, since they reduce the intensity of the main magnetic field. In view of the above reasons, the negative-direction current coils are necessarily used for the coils to form the innermost layer, i.e., the coils having an inner diameter, a central radius or an outer diameter as seen in their sectional views.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Other objects and advantages of the invention will become apparent from the following description of embodiments with reference to the accompanying drawings in which:

[0028]FIG. 1 is a sectional view schematically showing an NMR apparatus according to a first embodiment of the present invention;

[0029]FIG. 2 is a sectional view showing an arrangement of coils of a split type magnet device according to a second embodiment of the present invention;

[0030]FIG. 3 is a sectional view showing an arrangement of coils of a split type magnet device according to a third embodiment of the present invention;

[0031]FIG. 4 is a sectional view showing an arrangement of coils of a split type magnet device according to a fourth embodiment of the present invention;

[0032]FIG. 5 is a sectional view showing an arrangement of coils of a split type magnet device and a ferromagnetic member for shielding leakage magnetic field according to a fifth embodiment of the present invention;

[0033]FIG. 6 is a sectional view showing an arrangement of coils of a split type magnet device and a ferromagnetic member for shielding leakage magnetic field according to a sixth embodiment of the present invention;

[0034]FIG. 7 is a sectional view showing an arrangement of coils of a split type magnet device and a ferromagnetic member for shielding leakage magnetic field according to a seventh embodiment of the present invention;

[0035]FIG. 8 is a sectional view schematically showing a conventional NMR apparatus;

[0036]FIG. 9 is a sectional perspective view showing superconducting coils constituting the conventional NMR apparatus;

[0037]FIG. 10 is a sectional perspective view showing coils of a split type magnet device according to another embodiment of the present invention;

[0038]FIG. 11 is a block diagram showing the NMR apparatus of the present invention;

[0039]FIG. 12 is a diagram, for the purpose of explaining an effect of the present invention, showing a two-dimensional axisymmetrical distribution of currents;

[0040]FIG. 13 is a diagram, for the purpose of explaining an effect of the present invention, schematically showing a positional relationship between the current I which is represented by the position (f,α) and r,θ;

[0041]FIG. 14 is a diagram showing, for the purpose of explaining an effect of the present invention, a relationship between a second-order irregular magnetic field generated by the current I which is represented by the position (f,α) and the angle α;

[0042]FIG. 15 is a diagram showing, for the purpose of explaining an effect of the present invention, a relationship between a fourth-order irregular magnetic field generated by the current I which is represented by the position (f,α) and the angle α; and

[0043]FIG. 16 is a diagram showing, for the purpose of explaining an effect of the present invention, a relationship between a sixth-order irregular magnetic field generated by the current I which is represented by the position (f,α) and the angle α.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Specific embodiments of the present invention will hereinafter be described. FIG. 11 is a diagram schematically showing an NMR apparatus using a split type magnet. A superconducting magnet 39 formed of split coils is installed with a virtual central axis thereof extending along a substantially horizontal direction. A protein sample 40 is inserted to the apparatus from above, and a uniform static magnetic field is applied from the horizontal direction to the protein sample 40. Since a solenoid coil 41 is used for receiving an NMR signal, an SN parameter is improved by 40% as compared with that achieved by the conventional method wherein the saddle-like or the birdcage solenoid coil is used as mentioned in the foregoing. The superconducting magnet 39 is maintained at a permanent current mode by using a permanent current switch 42. The coils constituting the superconducting magnet 39 are connected with one another with the superconductive state being maintained by a superconducting connection 43. The superconducting magnet 39 is cooled and kept at a low temperature by a liquid helium stored in a liquid helium tank 44. The helium tank 44 is surrounded by a liquid nitrogen tank 45 to establish a dual structure of the helium tank 44 and the nitrogen tank 45, so that evaporation of the liquid helium is suppressed. The NMR apparatus as a whole is supported by vibration proof supports 46.

[0045] Next, FIG. 1 is a sectional view showing the superconducting magnet of the NMR apparatus according to the present invention, and FIG. 10 is a sectional perspective view of the superconducting coils. Each of superconducting coils 1 to 5 is made from a material having a high superconducting critical magnetic field. The closer the coils are to a sample 7, the higher the superconducting critical magnetic field. A protein sample 7 is inserted from above to be placed upright at the center portion of the magnetic field, and the magnetic field is applied to the sample from the lateral directions. Each of the superconducting magnets consists of a pair of left and right coils which are arranged symmetrically, and each of the coils is wound about a horizontal axis in a solenoidal fashion. A solenoid coil 6 which is made from copper and maintained at an ordinary temperature or a solenoid coil 6 which is Y-based or made from MgB₂ and cooled to 10 to 20 K is used for NMR signal detection. The superconducting magnets are set to a permanent current mode by a permanent current switch 10, and a time fluctuation of the magnetic field is adjusted to be 0.5 Hz per hour or less. The superconducting magnets are immersed in liquid helium 9 and maintained at a low temperature. A gap between a container containing the liquid helium 9 and a housing is filled with liquid nitrogen 8, so that the dual structure of the liquid helium 9 and the liquid nitrogen 8 contributes to conserving helium. The left and right superconducting coils of the present embodiment have central axes which are substantially concentric with each other in the horizontal direction and generate a horizontal magnetic field. The superconducting coils are arranged to be multilayered, and the left and right superconducting coils in each of the layers are substantially mirror-symmetrical with each other with respect to a centerline extending along the vertical direction of the magnet device. A direction of a current flowing through the superconducting coils 5 and 5′, which form the innermost layer together with the superconducting coils 4 and 4′, is reverse to that of currents of other superconducting coils. More specifically, the direction of the magnetic field generated in the sample area by the superconducting coils 5 and 5′ is reverse to the main direction of the magnetic field generated by the other superconducting coils. Owing to this structure, the split type magnet device shown generates a remarkably uniform magnetic field whose uniformity is the same as or superior than that generated by conventional multilayer solenoids.

[0046] Foregoing is a summary of the NMR system using the magnet of the split coil structure according to the present invention. Only a structure of a superconducting coil of the NMR system is described in each of the following embodiments.

[0047] An arrangement of superconducting coils according to a second embodiment of the present invention is shown in FIG. 2. Each of pairs of (left and right) superconducting coils 11 to 16 and 11′ to 16′ (ex., the pair of 11 and ll′) is arranged substantially concentric with each other with respect to a central axis extending along the horizontal direction. The left and right multilayer superconducting coils are arranged substantially symmetrically with respect to the centerline extending along the vertical direction of the magnet device, and a certain distance is kept between the centerline and each of the multilayer superconducting coils. The superconducting coils 14, 15, and 16 forms the innermost layer, wherein the superconducting coils 15 and 15′ are energized in such a manner that a magnetic field generated thereby is in the direction reverse to that generated at the central part. Thus, a magnetic field of ppb-order uniformity is formed at the center portion of the split type magnet device of the present embodiment.

[0048] An arrangement of superconducting coils according to a third embodiment of the present invention is shown in FIG. 3. Each of pairs of (left and right) superconducting coils 17 to 24 and 17′ to 24′ (ex., the pair of 17 and 17′) is arranged substantially concentric with each other with respect to a central axis extending along the horizontal direction. The left and right multilayer superconducting coils are arranged substantially symmetrically with respect to the centerline extending along the vertical direction of the magnet device, and a certain distance is kept between the centerline and each of the multilayer superconducting coils. Among the layers formed by the pairs of (left and right) superconducting coils, the innermost layers are formed of the superconducting coils 23, 23′, 24 and 24′ and layers disposed radially outward from the innermost layers are formed of the superconducting coils 20, 20′, 21, 21′, 22 and 22′. The pair of superconducting coils 22 and 22′ and the pair of superconducting coils 23 and 23′ are energized in such a manner as to generate a magnetic field which is in a direction reverse to that generated at the central part. Thus, a magnetic field of a ppb-order uniformity is formed at the center portion of the split type magnet device of the present embodiment.

[0049] The NMR apparatus should desirably have a small leakage magnetic field and, therefore, a fourth embodiment which will hereinafter be described includes shielding the leakage magnetic field.

[0050] An arrangement of superconducting coils according to the fourth embodiment of the present invention is shown in FIG. 4. Superconducting coils 26 to 30 generate a uniform magnetic field at the center portion of the magnet device. In the present embodiment, the superconducting coil 30 is energized to generate a magnetic field which is in a direction reverse to that generated by other coils, so that a magnetic field of ppb-order uniformity is generated. Superconducting shielding coils 25 and 25′ are active shielding coils used for suppressing the leakage of the magnetic field.

[0051] An arrangement of superconducting coils and a ferromagnetic member for shielding leakage magnetic field according to a fifth embodiment of the present invention are shown in FIG. 5. A cylindrical ferromagnetic member 31 and a disk-like ferromagnetic member 32 form magnetic paths to inhibit a magnetic field, which is generated by the superconducting coils, from leaking outside.

[0052] An arrangement of superconducting coils and a ferromagnetic member for shielding leakage magnetic field according to a sixth embodiment of the present invention are shown in FIG. 6. Superconducting shield coils 34 inhibit a magnetic field from leaking radially outward, while the disk-like ferromagnetic member 33 inhibits a magnetic field from leaking along the axial direction.

[0053] An arrangement of superconducting coils and a ferromagnetic member for shielding leakage magnetic field according to a seventh embodiment of the present invention are shown in FIG. 7. Superconducting shield coils 37 and 38 inhibit a magnetic field from leaking along the axial direction, while a cylindrical ferromagnetic member 36 inhibits the magnetic field from leaking radially.

[0054] Although all the coils constituting the magnet device are superconducting coils through the foregoing embodiments, the present invention is not limited to the embodiments and the coils may be those which can carry a current such as a coil fabricated by winding a copper wire. Further, a permanent magnet may be used as a source of the magnetomotive force for the static magnetic field source.

[0055] Various embodiments of the present invention are possible as described above; however, the invention is not limited to the embodiments described in this specification.

[0056] According to the present invention, the NMR apparatus for solution analysis using the split type magnet device can generate a magnetic field of ppb-order uniformity in a detection space. Since it is possible to insert a solenoid type probe coil into the magnetic field using a split gap of the magnet device, even an apparatus of 800 MHz can achieve a SN detection sensitivity equivalent to that of a conventional 1 GHz NMR apparatus. Further, since the intensity at the center portion of the magnetic field is relatively low, it is possible to shield the leakage magnetic field, thereby remarkably improving the installability of the apparatus. 

What is claimed is:
 1. A magnet device for NMR apparatus, comprising: first multilayer coils and second multilayer coils which face to each other with a predetermined distance being provided therebetween, each of pairs of the first and the second multilayer coils being substantially coaxial with respect to a central axis, each of layers of each of the first and the second multilayer coils having at least one coil, wherein an energizing current of at least one of the coils constituting an innermost layer of each of the first and the second multilayer coils is in a minus direction if an energizing current of the coil used for generating a main magnetic field for NMR detection in the vicinity of a center portion of the apparatus is in a plus direction.
 2. The magnet device for NMR apparatus according to claim 1, wherein an energizing current of at least one of the coils constituting layers other than the innermost layers of the first and the second multilayer coils is in a minus direction.
 3. The magnet device for NMR apparatus according to claim 1, wherein only the innermost layer of each of the first and the second multilayer coils is formed of a plurality of coils.
 4. The magnet device for NMR apparatus according to claim 1, wherein only the innermost layer and a layer or layers placed radially outward from the innermost layer of each of the first and the second multilayer coils are respectively formed of a plurality of coils.
 5. The magnet device for NMR apparatus according to claim 1, wherein the first multilayer coils and the second multilayer coils are arranged mirror-symmetrically with each other.
 6. The magnet device for NMR apparatus according to claim 1, further comprising a shield coil for shielding a leakage magnetic field or a ferromagnetic member for shielding a leakage magnetic field.
 7. The magnet device for NMR apparatus according to claim 6, further comprising a cylindrical ferromagnetic member and a shield coil.
 8. The magnet device for NMR apparatus according to claim 6, further comprising a disk-like ferromagnetic member and a shield coil.
 9. The magnet device for NMR apparatus according to claim 1, wherein each of the coils comprises a substance having superconductivity such that the magnetic device is capable of cooling the coils to or below temperatures at which the coils exhibit the superconductivity.
 10. An NMR apparatus comprising the magnet device for NMR apparatus of claim
 1. 